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What is Backlash?
One of the main reasons your CNC machine loses its accuracy, is due to- you got it- backlash! Backlash is the mechanical loss of motion that can result from a mechanical or electrical problem. Basically backlash is the amount the screw has to rotate when reversing directions before the table or turret start to move.
How is backlash compensated?
Backlash can be tested with indicators, lasers, ballbars or other measuring devices to determine how much motion is lost when an axis reverses direction. This value is entered in the machines parameters, so when an axis changes direction, it compensates the amount in the parameter to determine when the axis will physically move. The value of the parameter is usually in millimeter. Below, are the common parameters and program to run backlash test. Remember to Write down original values first!!!
Also, you may want to set the backlash comp first to zero to get the total amount of backlash to make sure it is not excessive. Reading should usually be below .008. Anything above .010 you might want to consider taking a closer look.
Where is the backlash coming from?
Make sure the bearings are pre-loaded properly. Run the each axis separately in a rapid program. Does it sound like a low growl? Compared to the other ones. If yes the bearings are bad. The problem could be any or a combination of the following- ballscrew or nut, bearings, ways, linear guides, belts, gears, coupling devices, and key stock.
NOTES: Write down original values in parameters first. Parameter value of 10 usually means .010mm on Fanuc controls. Other manufacturers may be different. 1mm=.0394 Also, some controls have a rapid and feed backlash, feed is usually all that needs to be adjusted.
Backlash Parameters
0 control X=535,Y=536, Z=537, 4th=538
3 control X=53, Y/Z=54, Z=55
6 control X=115, Y/Z=116, Z/3rd=117, 4th=118
10,11,12,15,16,18, 21,16I,18I,21I XYZ=1851
-2012
MX3 X=6400, Y=6401, Z=6402
LX3 X=6282, Y/Z=6283, Z=6284
Backlash explanation– Good article
http://www.maintenanceresources.com/Reference Library/ezine/backlash.htm
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Backlash Check Program
(do not use hand wheel)
G1 G98 G53 F10.
U.010
U-.010(record reading)
U-.010
U.010 (record reading)
M99
How to check backlash
The difference is your backlash.
Note: the larger the distance between moments during this test the more accurate it will be. Sometimes an axis must be jarred to make up excessive backlash and see more reliable results
Use with plunger style indicator and mount closest in-line with the ballscrew so, if there is any looseness or play in the gibs, this will have the least effect on your indicator readings. It is also a good idea to check farthest away from the center of the ballscrew to determine how much movement is coming from the gibs or linear guides. Change U or W values in the NC program to smaller values if you would like to use tenths indicator. For horizontal and vertical machines change the program above to use XYZ moves and use G91 and remove G98. G53 is a command that uses the machine coordinate for position. Your machine may not use this so you may need to alter it to fit your machine. You may also use the step mode to perform the test, but not the handwheel. Perform test in single block. Indicator must be firmly mounted, including stems to mag base in-line and retracted closest to the mag base for best results. If comparing with tenths indicator they should both read the same. If not you may see problems with stick slip effect. See below to do quick checks to determine this
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